A surface light source device having a two-dimensional light emitting surface is in widespread use, for example, as a backlight which is incorporated in a liquid crystal display device and illuminates a liquid crystal display panel from the back (see e.g. JP 2004-226503A). Surface light source devices for liquid crystal display devices are roughly classified into direct-light type devices which have a light source disposed right behind an optical member, and edge-light type devices (also called side-light type devices) which have a light source disposed lateral to an optical member.
Edge-light type surface light source devices have a structural feature that enables design of thin surface light sources as compared to direct-light type surface light source devices. Because of the structural feature, edge-light type surface light source devices have heretofore been applied principally in display devices for laptop personal computers (PCs).
Nowadays, with the development of light sources that emit light with high linearity, as typified by a light emitting diode, edge-light type surface light source devices have been put into use also in display devices having a larger size than laptop PCs, for example home televisions. However, it has turned out that when an array of light emitting diodes (LEDs) is used as a light source, in-plane variation of brightness (more precisely luminance) is produced in an area of a display surface (light emitting surface), lying in the vicinity of a light entrance surface that faces the light source. More specifically, in the area of the display surface (light emitting surface), lying in the vicinity of the light entrance surface, a pattern of alternating bright and dark portions is formed along the LED arrangement direction at the same pitch as the pitch of the array of LEDs.
It has also turned out that the in-plane variation of brightness is marked when a light guide plate is used which has linear prisms extending in the light guide direction and provided on the light exit side. In particular, when a light guide plate with prisms is used, in-plane variation of brightness is observed in a larger area of a display surface (light emitting surface).
It is expected that as the emission intensity of LED is improved in the future, a decreased number of LEDs will be arranged with a wider spacing in a light source in order to reduce the cost of the display device. It is, therefore, possible that the above-described in-plane variation of brightness may be observed more clearly in a wider area of a display surface (light emitting surface).